The invention relates to a DC-DC converter and, in particular, to a Buck converter.
Often space limitations in a device do not allow multiple batteries to supply different parts of the device. As a result, DC to DC converters are important in mobile devices such as cellular phones and laptop computers receiving power from batteries.
Efficiency is a major concern in DC-DC converter design. DC-DC converters with a digital control loop are popular because of flexibility. With programmable gain and loop bandwidth, it is easy to program the DC-DC converter according to different LC filters or switching frequencies, etc.
FIG. 1 is a schematic diagram of a conventional DC-DC converter with a digital control loop. The DC-DC converter comprises an output stage and a digital controller. The output stage comprises a PMOS transistor, an NMOS transistor, and an LC filter. The PMOS and NMOS transistors are series connected between a power supply PVDD and a ground PGND. The LC filter comprises an inductor, coupled between a common drain of the PMOS and NMOS transistors and an output node of the DC-DC converter, and a capacitor, coupled between the output node and the ground PGND. The output voltage of the output node is sampled by a voltage divider, such as a resistor in FIG. 1. The digital converter comprises an A/D converter and a digital control circuit powered by a digital power supply DVDD. The A/D converter is coupled to the voltage divider and receives the sampled output voltage. The sampled output voltage is converted to a digital control code and transmitted to the digital control circuit. The digital control circuit controls switching of the PMOS and NMOS transistors according to the digital control code.
One problem with this DC-DC converter is that power for digital and analog circuits in an integrated circuit is typically supplied by different sources. For example, in a device powered by a lithium-base battery, a voltage of 1.8V, for system digital power, down-converted from 3V is often required. While 3V power the analog circuit in DC converter, the digital controller requires 1.8V and cannot be directly connected to the battery. This problem can be eliminated using an additional low dropout regulator to provide digital power, as shown in FIG. 2. However, efficiency for LDO depends on the output voltage to battery voltage ratio, resulting in efficiency loss and increased chip area.